The ready availability of commercial red and infra-red light emitting diodes and laser diodes for imaging systems has stimulated the development of imaging materials sensitive to the output wavelengths of these devices. Imaging materials may be broadly grouped into either silver halide or non-silver halide based materials. Whereas silver halide based materials have frequently been sensitised to the infra-red, non-silver halide materials (with the exception of electrophotographic materials) have rarely been sensitised to the infra-red. A common form of non-silver imaging involves photocuring of unsaturated resins. This is used extensively in printing and proofing, for example, by exploiting the different physical properties of the cured and uncured resin. Images are revealed, e.g., by differential dissolution in developer, by differential adhesion of coloured toner, by differential adhesion to a carrier sheet etc. Normally, such materials comprise a suitable resin, an initiator and a sensitiser, although the last two may be combined in a single molecule. The function of the sensitiser is to absorb incident radiation and transfer energy to the initiator, which then fragments into reactive species that initiate curing of the resin.
The most commonly used sensitisers respond to relatively short wavelength (high energy) light, e.g., 350 to 420 nm, so that relatively large amounts of energy per absorbed quantum can be transferred to the initiator. The latter can therefore be a thermally stable compound, with good shelf-life. Examples of such systems are found in, for example, British Published Patent Applications Nos. 2111232A, 2112536A and 2113860A. At longer wavelengths, the energy available per absorbed quantum decreases correspondingly, until there is insufficient energy to cause fragmentation of any but the most unstable of initiators, which then have a very limited shelf life.
Japanese Patent Applications Nos. 63-208036 and 63-274592 disclose the combination of infra-red absorbing polymethine or squarylium dyes with an organic peroxide and curable resin. Heat generated through absorption of infra-red radiation causes decomposition of the peroxide and curing of the resin, the materials being imageable by semiconductor laser.
European Published Patent Application No. 223857 and Japanese Patent Applications Nos 1-013139 to 1-013144 disclose the combination of a cationic dye and organoborate salt as photoinitiator. Sensitivity throughout the visible region is possible, but infra-red sensitivity is not disclosed.
Iodonium salts are known as initiators in photo-curing systems. They are most commonly sensitised to the 350 to 420 nm region, but recent publications such as U.S. Pat. No. 4,828,583 and European Published Patent Application No. 290133 disclose sensitisation up to 660 nm. These publications also teach the additional benefit of including an electron donor compound having an oxidation potential greater than zero but less than or equal to that of p-dimethoxybenzene. Improvements in the rate and depth of cure are described. In both U.S. Pat. No. 4,828,583 and European Published Patent Application No. 290133, the sensitising dyes are restricted to those materials which sensitise 2-methyl-4,6-bis(trichloromethyl)-s-triazine.
The combination of iodonium salts and oxonol dyes is disclosed in U.S. Pat. Nos. 4,701,402 and 4,632,895. The systems described are photobleaching and there is no disclosure of photopolymerisation.
A class of dyes, particularly oxonol dyes, has now been found which have a particular utility as photosensitiser dyes in photocurable polymer based imaging systems.